1. Field of Invention
The present invention relates to a metallic ingot for plastic working and a method for producing the same. More particularly, the present invention relates to forging stock of such as aluminum or the like for cold-forging, hot-forging and closed forging. Metal, to which the present invention can be applied, is non-ferrous metal such as aluminum, zinc and magnesium and the respective alloys, as well as ferrous materials. Metals particularly suited for the ingot of the present invention are aluminum, zinc and magnesium. Aluminum is described hereinafter as a representative metal.
2. Description of Related Art
Usually, an extruded or continuously cast bar is cut to a requisite length and width and is used for forging stock (c.f. Japanese magazine "Alu" July, 1995 published by Light Metal Communication Co., Ltd., Jul. 28, 1995, pages 33 and 34). More specifically, in the case of an extruded bar, aluminum melt is continuously cast to form a small-diameter bar, which is then annealed and scalped. The bar is then cut to a predetermined length or width. A bar having an irregular cross section or a hollow bar may be cut to a predetermined thickness.
A rolled sheet is blanked to a round disc and is used for the forging stock. More specifically, aluminum melt is continuously cast to form rolling stock which is heated and then hot-rolled to form a rolled sheet. It is then blanked by a blanking machine to a predetermined diameter so as to provide the forging stock.
In addition, the melt is directly continuously rolled to a sheet, which is then blanked to provide the forging stock.
The forging stock provided by the above described methods has a cut, machined or plastically worked surface and is hence not an ingot itself, i.e., the cast material entirely having a cast surface.
There are also methods for obtaining an ingot, such as metallic-mold gravity casting, die casting and low- or high-pressure casting and the like. Aluminum melt is poured in a casting apparatus to form an ingot, whose sprue, riser and the like are cut when the ingot is to be forged.
The aluminum Forging Committee organized in The Light Metal Association of Japan carried out research on the so-called "Casting and Forging Method", in which melt is filled in all portions of a mold corresponding to the respective portions of a forging pre-form, and further, solidification speed of the melt in all portions is controlled to an optimum level so as to prevent the defects. This method can be said to be an improvement of the metallic-die gravity casting and die-casting. However, in order to forge the resultant ingot, the sprue, riser and the like must be cut off (c.f., Alu idem, page 42).
Apart from the above methods, a unidirectional casting is known in the casting of steel (Japanese Unexamined Patent Publication No. 56-50776).
An experimental plant for the unidirectional casting is known in the field of aluminum alloy (c.f., Japanese magazine "Foundry", Vol. 49 (1977), No. 9, pages 539-544). A sketch of the plant is shown in FIG. 2. A mold 2 is placed on the cooling plate 1 provided with a water cooling nozzle 10. The melt 7 poured in the mold is cooled by the cooling plate 2 so as to unidirectionally advance the solidification interface 12 in the direction of the arrow or vertically upward. In FIG. 2, the top cover is denoted by 8. The electric furnace 9 prevents melt 7 and an ingot 2 from being preferentially cooled on the sides.
The cast and then extruded ingot, and the continuously cast and then cut bar have good internal quality but are produced through a complicated process, while requiring considerable man-hours for working. In addition, aluminum scraps are generated in quantity in the course of the production process, with the result that the yield is lowered and hence the production cost increases. However, the total competitive advantage of the extruded material and continuously cast and then cut bar is overwhelmingly higher than the other forging stocks with respect to cost and quality. Extruded material and continuously and then cut material account for the majority of the aluminum forging stock.
The cost of forging stock produced by blanking the rolled sheet is high for the reasons as described for the extruded material and the like. Moreover, it is difficult to produce all the forging stock of alloy-grades, whose rolling is difficult.
The direct rolling method has been developed to lower the rolling cost. However, since the direct rolling of high-strength aluminum-alloys is difficult, the applicable alloy grades are more restricted as compared with the ordinary rolling. The direct rolling method is therefore not generally practical.
The ingots can be produced by a simple process by means of the metallic-mold casting, die-casting, high-pressure or low-pressure casting, and the like. The production cost is therefore low as compared with the continuously cast bar and the wrought materials. The ingots always include, however, such defects as casting cavities, solidification segregation, pin holes, shrink cavities, and oxide inclusions. When solidification advances due to heat withdrawal of a mold, the solidification interfaces advance from all walls of the mold and collide with one another in the final period of the solidification. The impurities, gases and the like are therefore left at a location where the solidification completes and defects generate. Even if an ingot has a simple shape as in the case of a forging stock, it is difficult to take measures to avoid the defects when the thickness is small as compared to the diameter of an ingot because oriented solidification is difficult. Since the cast forging stock, produced by the above described casting methods, has numerous defects, it is difficult to manufacture by such forging stock structural parts which are required to exhibit a particularly high level of mechanical strength and fatigue strength. If such forging stock is applied, it must be strictly inspected for quality, which increases the inspection cost and lessens the yield of product. The total cost of the finished parts is higher than that of the forging stock.
Meanwhile, defects of the unidirectionally cast ingot are few. This ingot has not been used as the forging stock. The present inventors considered whether or not such ingot could be applied for forging.
The quality of a unidirectionally cast ingot is good. However, since the top surface of the melt is open and freely solidifies, the meniscus part of the melt being in contact with the mold is largely curved, and solidifies with curvature R as shown in FIG. 3(a). Contrary to this, the extruded bar or continuously cast and then cut bar has a rectangular peripheral surface as shown in FIG. 3(b). Since the radius (R) of the meniscus greatly varies depending upon the melt temperature, the pouring method of melt into a mold, vibration of a mold, and the other factors, the shape of an ingot considerably varies. Particularly in the case of die-forging stock, its shape is greatly influenced by the finishing of forgings. When the forging stock is thin or the product has a complicated shape, the influence of meniscus curve on forging is not negligible. Therefore, forging stock having the meniscus must be loaded in a forging die so that the surface having the meniscus is predetermined with either the die bottom or situated above. The loading direction of a forging stock surface must be predetermined in view of influence of the meniscus upon the finishing of forged product. The forging stock cannot therefore be loaded in the forging dies such that the top of ingots may be directed in any direction. Such limitation makes the application of the forging stock impractical.
In addition, it is difficult to control the pouring to a constant amount, with the result that: weight of the forging stocks disperses; the forging machine stops due to overload applied to the same; and, weight and shape of the forged product disperse greatly. It is therefore difficult to produce according to the prior art the forging stock having improved internal quality, high dimensional accuracy and high weight accuracy.